Try this:
var zipcode = 'zip99999'; // or 'zip11111'
$('#textbox2city').val( data[zipcode] );
and
var zipcode = "zip" + $("#textbox1zip").val();
$('#textbox2city').val( data[zipcode] );
Now, this alert( data.zipcode ); -----> returns 'undefined'
This happen since the json object you have only two keys zip11111
& zip99999
. So, data.zip11111
or data.zip99999
works, whereas data.zipcode
doesn't since there's no key with the name zipcode
.